1. Field of the Invention
This invention relates to a spotting tip which is fitted on a liquid sucking nozzle for sucking liquid in a container, and more particularly to a spotting tip which sucks liquid to be subjected to a biochemical analysis or the like and spots the liquid on an element, device or the like for biochemical analysis.
2. Description of the Related Art
Quantitative analysis of a particular chemical component in a sample liquid is commonly carried out in various industrial fields. For example, there have been known various methods of potentiometrically analyzing an activity (or concentration) of a particular ionic substances contained in a spotted sample liquid such as liquors, beverages, city water or a body fluid such as blood, urine or saliva. Ionic activity measuring devices for measuring an activity of an ionic substance in a spotted sample liquid are disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 59(1984)-30055 and 6(1994)-66759, and U.S. Pat. No. 4,437,973.
Generally such an ionic activity measuring device includes at least a set of ion selective electrode pair each having an outermost layer composed of an ion selective layer which selectively responds to a particular ion such as Na, K, Cl or the like. The ion selective electrode pair are supported between an upper frame and a lower support frame, and a pair of liquid spotting holes are provided in the upper frame respectively opposed to the electrodes. A porous bridge which establishes an electrical connection between a sample liquid spotted to one of the spotting holes and a reference liquid spotted to the other spotting hole is provided in the upper frame. Preferably the porous bridge is formed of twisted yarn of fibers.
In such an ionic activity measuring device, when a reference liquid containing therein an ionic substance whose ionic activity is known is spotted to one of the spotting holes and a sample liquid containing therein the ionic substance whose ionic activity is unknown is spotted to the other spotting hole, the liquids soak into porous liquid distributing members and reach the corresponding ion selective electrodes while the interfaces of the liquids are brought into an electrical contact with each other near the middle of the porous bridge. As a result, there is produced a potential difference between the electrodes according to the difference between the activity of the ionic substance in the sample liquid and that in the reference liquid. By measuring the potential difference, the activity of the ionic substance in the sample liquid can be determined on the basis of a calibration curve or a standard curve which has been determined in advance on the basis of Nernst equation.
The ionic activity measuring device is very useful for analysis of aqueous sample liquid, especially for clinical analysis of a sample liquid from a human body such as blood since the ionic activity can be easily determined by only spotting droplets of the sample liquid and the reference liquid.
When spotting the sample liquid and the reference liquid, the liquids are sucked in different spotting tips and dropped onto the respective spotting holes from the ends of the tips. Each tip is fitted on a liquid sucking nozzle and dipped into the sample liquid or the reference liquid and then the sample liquid or the reference liquid is sucked into the tip in a predetermined amount in response to a sucking operation of the liquid sucking nozzle and then held there. Then the tips on the sucking nozzles are moved to predetermined positions above the ionic activity measuring device and then discharge the sample liquid and the reference liquid onto the spotting holes in response to a discharging operation of the nozzles. The sample liquid and the reference liquid may be spotted either simultaneously or in sequence generally within 60 seconds.
In the container containing the sample liquid or the reference liquid, when the mouth of the container through which the spotting tip is inserted into the container is kept open for a long time, there is a possibility of the liquid evaporating to change the concentrations of the components and spoiling.
Accordingly conventionally the mouth of the container is normally covered with a lid and is opened only when the liquid therein is to be sucked. The lid is put on again immediately thereafter.
However shutting and opening the mouth is very troublesome and puts an obstacle in automating the liquid sucking operation.
In order to overcome the problem, we have proposed a novel tip in our Japanese Unexamined Patent Publication No. 7(1995)-80331. The tip is provided with a lid member which is brought into contact with the mouth of the container and closes the space between the tip and the edge of the mouth when the tip is inserted into the container to a predetermined depth. The tip is normally held in the container with the lid member in contact with the mouth except when the liquid in the container is to be spotted, whereby the container is held air-tight and the liquid in the container is suppressed from evaporating and diffusing outside.
However when the tip was actually used, the liquid in the container flowed back into the tip and in extreme case even into the nozzle after the tip was held in the container for a long time. When such a back flow of the liquid occurs, the liquid cannot be spotted in an accurate amount due to liquid remaining in the tip or nozzle and there is a possibility of the concentration of the liquid changing.
Though not clear, the reason why the back flow occurs is considered to be that when the temperature in the container is raised while the container is held air-tight by the lid member for a long time, gases including vapor expand and raise the liquid in the container into the tip or the nozzle.